The mechanisms involved in generating the large numbers of precise connections between neurons are unknown. In order to find and eventually synapse with their appropriate target cells, developing neurons choose stereotyped routes, often travelling long distances and bypassing many cells. During insect embryogenesis, the growth cones of individual neurons actively recognize and grow along particular axon surfaces, suggesting the expression of a number of cell-surface molecules mediating these recognition events. The long-term goal of this proposal is to understand the molecular bases of these recognition events. To this end, a most promising approach is to isolate mutations which alter the recognition events and in this way identify the genes and gene products involved. Recently, we have characterized in detail the interaction and recognition between single identified neurons in the Drosophila embryo, with the goal to capitalize on its advanced genetics and molecular biology to extend our analysis to the molecular level. The specific aims of this proposal are 1) to isolate single-gene point mutations in Drosophila which alter the normal stereotyped patterns of recognition between embryonic neurons, 2) to isolate point-mutations, and thus identify the relevant genes, within two chromosomal regions which have been shown to disrupt the patterns of recognition when deleted, and 3) to initiate the molecular cloning of the genes of interest. Mutations will be identified and analyzed by using specific antibodies to the Drosophila nervous system and by Lucifer Yellow dye injections of individual neurons. The molecular cloning of genes will be initiated by obtaining P-element insertional mutations.